1. Field of the Invention
The present invention relates to spinal disc prostheses, methods of placing spinal disc prostheses between adjacent vertebrae and surgical instruments used in connection with the same.
2. Description of the Prior Art
The vertebrae of the human spine are each separated by a relatively soft disc which acts as a joint that allows the spine to flex, extend, bend laterally and rotate. Each disc includes a tough outer fibrous ring that binds the adjacent vertebrae together. The fibrous portion consists of overlapping multiple plies that are attached to the vertebrae in a manner that resists torsion whereby half of the angulated fibers making up the ring will tighten when the vertebrae rotate in either direction relative to each other. The inside of the disc has a high water content which aids in the load-bearing and cushioning properties of the disc.
Unfortunately, one or more discs in the spine can be displaced or damaged due to trauma or disease. A disc herniation occurs when the fibers are weakened or torn and the disc becomes permanently stressed, extended or extruded out of its normal confines. A herniated or slipped disc can compress a spinal nerve resulting in pain, loss of muscle control, or even paralysis. Also, the disc degeneration causes it to lose water and deflate. As a result, the height of the disc decreases causing it to buckle. As the buckling takes place, radial or annular tears may occur and contribute to persistent and disabling pain.
While a variety of disc prostheses and placement methods have been developed and used heretofore, they often involve fixed rigid approaches and systems which do not restore normal function and/or require long and complex operations. Flexible and relatively dynamic prostheses have been developed; however, many of such prostheses are too large to be inserted around the spinal nerves therefore cannot be placed using a posterior approach (through the back of the patient). Anterior approaches to disc replacement (through the abdomen of the patient) can be more difficult due to many factors, including possible injury to major blood vessels, a risk of impotence and other complications, and the inability to directly see pinched nerve elements. The assistance of a thoracic or vascular surgeon is often required when an anterior approach in utilized.
Thus, there is a need for a lumbar spinal disc prosthesis that can be easily placed between adjacent vertebrae using a posterior approach, that is relatively simple and that provides normal spine function without pain or disability. There is also a need for an improved method of placing a spinal disc prosthesis between adjacent vertebrae based on a posterior approach. Finally, there is a need for improved surgical instruments and apparatus for carrying out the placement procedure.